Function:     cubicspline

Purpose:      Cubic spline through X and Y positions.
                            
Category:     MATH
                           
File:         cubicspline.c
                          
Author:       M.G.R. Vogelaar
                        
Description:  Piecewise cubic spline trough 'np' two dimensional positions
              stored in arrays 'X' and 'Y', using 'cord' approximation.
              See 'Mathematical Elements for Computer Graphics', 2nd ed. 
              David F. Rogers, J. Alan Adams. p. 262.

Use:          void cubicspline( float  *Xus,
                                float  *Yus,
                                int    np, 
                                float  sampdist,
                                float  **XusO,
                                float  **YusO,
                                int    *nout,
                                fint   *blo,
                                fint   *bhi,
                                float  *splineleningrids )
              
              On input the variables Xus and Yus contain the contol points of 
              the spline. The number of points is 'np'.
              'XusO' and 'YusO' are pointers to arrays and are set to NULL.
              In the function 'cubicspline' memory is allocated for these arrays. 
              You need to free memory in the calling environment.
              
              On output, these arrays 'XusO' and 'YusO' contain 'nout' positions that are 
              interpolated along a cubic spline on a sample distance given 
              by 'sampdist' (usually 1). 
              Also the length of the spline in grids is returned ('splineleningrids').
            
Remarks:      
              
Updates:      Jul 26, 2004: VOG, Document created.